The invention relates to AM stereo radio broadcast receivers and is particularly directed to a receiver using the Magnavox system recently announced as the system of choice by the Federal Communications Commission. In this system, the conventional amplitude modulated (AM) radio channel carries the L+R stereo signal so that a conventional monaural radio receives a compatible signal. The L-R stereo signal is transmitted as a phase modulation (PM) of the carrier. A subaudible pilot tone also phase modulates the carrier. Its phase modulation is substantially greater than the L-R component. Since the conventional radio will not respond to the PM, it will not be affected thereby. However, if a limiter and PM detector are added to a conventional radio, the AM will be ignored and the PM can be recovered. Therefore the L-R information and pilot signal can be separately recovered. It is then only necessary to matrix the two channels to recover the stereo signals for reproduction in a stereo audio system.
While the circuits disclosed herein are intended for use with the proposed Magnavox system, it is to be understood that the functions performed can be used with other proposed AM stereo systems.
It is important that the additional stereo receiver circuitry be available in integrated circuit (IC) form to minimize the economic impact of AM stereo on receiver construction. It is also important that the decoder does not produce signal radiation that can be picked up by the radio signal circuits where it can produce spurious responses. It should be as linear as possible, noise free, and as immune to overmodulation and carrier propagation problems as possible.
In most receiver proposals currently available, the PM detection systems employ a synchronous detector and a phase locked loop oscillator (PLLO). Such an arrangement commonly produces substantial radiation and is excessively responsive to overmodulation of the carrier.